The Syntheses and Vibrational Spectra of 16 O-and 18 O-Enriched cis -MO 2 (M = Mo, W) Complexes

: In this contribution, we report convenient synthetic approaches for obtaining 16 O/ 18 O-enriched dioxidometal VI complexes, MO 2 (L) (W, Mo), with a linear, tetradentate amine phenolate ligand N , N '-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H 2 L) and describe their characterization by IR and Raman spectroscopy complemented by DFT computational analysis. The isotopologues of WO 2 (L) were made of tungsten VI trisglycolate W(eg) 3 (eg = 1,2-ethanediolate dianion) and ligand H 2 L in the presence of either H 2 [ 16 O] or H 2 [ 18 O], whereas Mo 16 O 2 (L) was made using Na 2 MoO 4 ·2H 2 O which was converted to Mo 18 O 2 (L) by oxido substitution using H 2 [ 18 O]. The complementary IR and Raman analyses show the ν (MO 2 ) s and ν (MO 2 ) a at 934 and 899 cm –1 for W 16 O 2 (L) and at 914 and 898 cm –1 for Mo 16 O 2 (L), respectively. In the vibrational spectra of the 18 O substituted derivatives, the ν (MO 2 ) s were shifted to lower energy by 43 cm –1 for W 18 O 2 (L) and by 41 cm –1 for Mo 18 O 2 (L) whereas asymmetric MO 2 stretches in the IR were partially overlapped by an organic ligand related stretch. However, Raman spectroscopy, accompanied by DFT calculations, allowed the deciphering the ν (MO 2 ) a shifts of 47 cm –1 for W 18 O 2 (L) and 31 cm –1 for Mo 18 O 2 (L).


Introduction
A number of dioxidotungsten VI and -molybdenum VI compounds have been prepared for catalytic applications in important industrial processes, such as olefin epoxidation. [1,2] uch complexes are also considered as soluble molecular models for metal oxide catalysts. [3]Moreover, tungsten and molybdenum are found in nature as water-soluble and biologically available highvalent oxido species and are used by a variety of metalloenzymes that take part in oxygen atom transfer (OAT) reactions.In such reactions, an oxygen atom is transferred from an oxygen donor to a biologically relevant acceptor molecule or vice versa. [1,2,4,5] Be8][9][10] We have previously synthesized a number of tungsten VI complexes with amine bisphenolate ligands using tungsten VI alkoxide W(eg)3 (eg = ethylene glycolate dianion) as a precursor and found that the number of terminal oxido ligands in the formed complex depends on the denticity of the amine bisphenol.13] However, the origin of these terminal oxido ligands in the reactions of a trisglycolate precursor is not unambiguous.Water is the most likely source of the oxo-groups, but, on the other hand, it is known that high-valent molybdenum, tungsten and rhenium alkoxides can decompose by an elimination process, which yields an ether and the corresponding oxidometal species. [14,15] e decided to investigate the mechanism of the formation of the oxido species by reacting W(eg)3 with an amine bisphenol ligand in the presence of either H2[ 16 O] or H2[ 18 O] to form a dioxido complex WO2(L).If the formation of the WO2 2+ moiety occurs due to ether elimination, W 16 O2(L) is expected to form regardless of whether H2[ 16 O] or H2[ 18 O] is used, whereas the hydrolytic cleavage of the alkoxide bonds should lead to the formation of W 16 O2(L) and W 18 O2(L), correspondingly.These two isotopologues can be easily differentiated by the mass spectroscopy and are expected to have distinctive vibrational spectra as well.For the investigation of the 16 O/ 18 O labelling in the synthesized dioxido complexes we selected IR spectroscopy which is a powerful tool in probing the cis-MO2 (M = Mo, W) unit as it usually generates two characteristic strong to medium absorption bands around 900 cm -1 in the IR spectrum.Since the pioneering work by Griffith in the late 1960´s, these IR absorptions are generally assigned as symmetric and asymmetric O=M=O stretches, respectively. [16,17] pecifically, molecular complexes [MoO2(L n )] show two distinctive absorption maxima at 915-935 and 890-905 cm −1 , respectively, whereas complexes [WO2(L)n] display typically two strong vibrational bands at ca. 930-960 and 870-920 cm −1 in their IR spectra. [4, 6-10, 12, 18-22]Generally, these two bands show a difference of 20-35 cm -1 .For tungsten complexes, the former bands usually occur at 10-25 cm −1 higher frequencies than those of their molybdenum congeners.For example, the resonances ν(MO2)s and ν(MO2)a for [MO2(ONNO H )] (M = Mo, W; H2ONNO H = N,N'-bis(2-hydroxyphenyl)ethane-1,2-diamine) are seen as strong absorptions at 937 and 916 cm -1 for Mo and at 951 and 911 cm -1 for W complexes, respectively. [7]o date, numerous dioxidotungsten VI and -molybdenum VI complexes with tetradentate amine phenolate ligands have been prepared and their IR spectra have been reported. [6,7,12,23,24] We ave earlier prepared the structurally identical complexes of tungsten and molybdenum using N,N′-bis(2-hydroxy-3,5dimethylbenzyl)-N,N′-dimethylethane-1,2-diamine (H2L) ligand with a natural isotope distribution (Figure 1). [13,25,26] Bth these complexes show four strong to medium intensity absorptions in the region typical for the cis-MO2 moiety.These are 959(m), 934(vs), 899 (vs) and 868(m) cm -1 for WO2(L) and, respectively, 949(m), 914(s), 903(vs) and 868(m) cm -1 for MoO2(L).Following the generally accepted definition, the strong bands at 934 and 899 cm -1 for WO2(L) and corresponding bands at 914 and 903 cm -1 for complex MoO2(L) were assigned as ν(MO2)s and ν(MO2)a, respectively.However, at the time it was not possible to unambiguously verify the correct assignment of the vibrational spectra.A significant aspect in the present study is to use the 18 O labelling of the terminal oxido-groups to acquire more evidence for the assignment of abovementioned absorption bands.Therefore, we have prepared not only the 18 O labelled MO2(L) complexes of tungsten VI but also molybdenum VI and compared their vibrational spectra with the complexes having a natural isotope distribution.In addition, the established reaction routes for the M 18 O2(L) complexes allowed us to investigate the lability of the terminal oxide ligands in the WO2(L) complex in respect to the analogous MoO2(L) complex.

Syntheses and Reactivity
The tungsten compounds W 16 O2(L) and W 18 O2(L) used in this study were prepared following the known procedure, i.e. by the reaction of W(eg)3 (eg = ethylene glycolate dianion) with the ligand precursor in the presence of H2[ 16 O] or H2[ 18 O] (Scheme 1a).In principle, the formation of a WO2 2+ moiety using trisglycolate precursor may be due to the ether elimination process of the alkoxide groups [14,15] or by hydrolysis.The mass and IR spectra (see below) show that the addition of either H2[ 16 O] or H2[ 18 O] in the reaction mixture leads to the clean formation of the respective isotopologue which demonstrates that the formation of dioxidotungsten centre is caused by the hydrolysis reaction wherein the included water acts as the oxide source.The molybdenum complex Mo 16 O2(L) was prepared by the reaction of sodium molybdate with the ligand precursor H2L in an acidic methanol solution. [26]The 18 O enriched complex Mo 18 O2(L) was obtained straightforwardly by heating Mo 16 O2(L) with an excess of H2[ 18 O] in methanol solution and allowing the complex to crystallize upon cooling (Scheme 1b).The tungsten complex W 16 O2(L) can be made of sodium tungstate following the identical procedure applied for the preparation of Mo 16 O2(L).However, the oxygen atom exchange with H2[ 18 O] was found unproductive.All products were identified by 1 H and 13 C NMR spectroscopy, which show expected chemical shifts for the octahedral complexes with C2 rotation axis. [13,25] he isotopic purity of the compounds was unequivocally confirmed by mass spectrometry.We also studied the possibility of structural polymorphs forming upon the 18 O substitution, but the measured powder X-ray diffraction patterns (PXRD) of all four studied complexes demonstrated the isostructural nature of the bulk samples and their distinctive correspondence to the simulated PXRD patterns of the respective single crystal structures [25,26] (see Figure S1 in the Supporting Information).

Spectral characterization
The general features of IR spectra of all studied compounds are very similar apart from strong absorptions around 900 cm -1 ; i.e. the anticipated spectral region of the cis-MO2 related absorptions (Figure 2).The IR frequencies for the cis-MO2 related absorptions are expected to show shifts to the lower energy upon 18 O substitution assuming that the lattice parameters and atomic positions remain the same.It was also anticipated that the isotope substitutions would shift the frequencies of both symmetric and asymmetric cis-MO2 stretches roughly 5% ([v(Mo= 16 O)/v(Mo= 18 O)] = 1.051) [27] , that is ca.30][31][32][33] These presumptions in mind we carried out an initial assignment of the ν(MO2)s and ν(MO2)a modes as follows: for the complexes with natural isotope distribution the symmetric and asymmetric cis-MO2 stretches were found at 914 and 898 cm -1 in case of Mo 16 O2(L) whereas for W 16 O2(L) the respective IR absorption bands were found at 934 and 899 cm -1 .While these absorption bands were straightforward to decipher we experienced difficulties in identifying the ν(MO2)a modes of the 18 O enriched complexes.The parameter  = ν(MO2)s -ν(MO2)a (cm -1 ) can be used to position these absorption bands; in related cis-MO2 complexes  is typically ca.33 cm -1 for tungsten [24] and ca.22 cm - 1 for molybdenum. [5]The ν(MO2)s bands for Mo 18 O2(L) and W 18 O2(L) were seen at 873 and 891 and cm -1 , respectively, but   the corresponding ν(MO2)a bands, which were expected to emerge in the 840-870 cm -1 region, were obscured by strong absorptions arising from the bisphenolate-metal O-M-O modes.
As a result, we were unable to reliably determine these asymmetric M 18 O2 stretches from IR spectra alone.
To further extend our analysis of the vibrational modes we recorded the Raman spectra of all studied compounds.Due to the approximate C2 symmetry of the complexes, we expected to see all the vibrational modes both IR and Raman active but the difference in the relative intensities of the peaks given by the two methods could provide useful information upon the correct assignment of the cis-MO2 stretching modes.The observed Raman spectra exhibit nearly identical modes compared to IR in the 900±50 cm -1 area, but show significant differences in their relative intensities (Figure 3).Especially, Raman intensities of ν(MO2)s modes appear much greater than ν(MO2)a which is in stark contrast to the corresponding IR vibration intensities since the symmetric stretch induces a greater change in the dipole moment compared to the asymmetric stretch.The measured Raman spectra therefore support the assignment of 914 and 873 cm -1 to the ν(Mo 16 O2)s and ν(Mo 18 O2)s modes, respectively, whereas the corresponding symmetric stretching modes of W analogues are observed at 933 and 891 cm -1 .This assignment of ν(MO2)s also supports the one done according to the IR spectra (vide supra).
To aid in interpreting the experimental IR and Raman spectra we conducted a computational analysis of the natural and  geometries were used to produce the corresponding IR and Raman spectra of 16 O and 18 O analogues of both Mo and W complexes (Figure S2, see further information in computational details).The geometries exhibit C2 symmetry and thus all calculated vibrational modes are both IR and Raman active and the calculated IR and Raman spectra of corresponding complexes differ only in their relative intensities.The spectral data showed no significant mixing of the cis-MO2 stretching with other modes thus it was possible to unambiguously assign the symmetric and asymmetric cis-MO2 stretching frequencies of all complexes from the calculated spectra.The relevant calculated vibrational data is shown in Table 1 The general tendency of PBE functional to underestimate the vibrational frequencies results in a good overall agreement between the experimental and calculated frequencies without an external scaling factor.For the Mo 16 O2(L) complex the calculated ν(MoO2)s modes appear at 948 and 900 cm -1 (experimental 914 and 873 cm -1 , in IR) whereas for Mo 18 O2(L) the ν(MoO2)a modes are calculated to be at 936 and 894 cm -1 (experimental 897 and ca.860 cm -1 , Raman).The calculated differences () of 12 and 6 cm -1 between the symmetric and asymmetric MoO2 stretching modes of Mo 16 O2(L) and Mo 18 O2(L) support that the shoulder, observed in the Raman spectrum of Mo 18 O2(L) at about 860 cm - 1 , corresponds to the ν(MoO2)a mode.
In the case of W 16 O2(L), the ν(WO2)s is calculated to be 947 cm -1 (experimental 934 cm -1 ) and ν(WO2)a is 918 cm -1 (experimental 900 cm -1 ).For W 18 O2(L), the Raman spectrum clearly reveals the ν(WO2)s band at 891 cm -1 , even if it is more difficult to assign the ν(WO2)a mode.However, according to the calculated W 18 O2(L) frequencies the ν(WO2)a mode should be red shifted ca.26 cm -1 to roughly 865 cm -1 .Indeed, this region shows changes upon 18 O substitution in both experimental vibrational spectra but particularly in Raman wherein a peak value of either 863 or 872 cm -1 can be suggested for the asymmetric stretch.Although we cannot unambiguously assign either of the two to ν(W 18 O2)a mode, the proposed interpretation indicates that the absorptions at 909 cm -1 and 925 cm -1 in the spectra of 18 O substituted Mo and W complexes, respectively, are not related to cis-MO2 modes.

Conclusions
The The IR and Raman spectroscopic studies showed that although plenty of earlier experimental data of MoO2 and WO2 is available, which aids in characterization of their analogous complexes, the assignment of their symmetric and asymmetric cis-MO2 stretching modes is not trivial.After recognizing the partial overlapping of ν(WO2)a modes with vibrational modes arising from the ligand backbone, we were able to show, by using experimental IR and Raman and computational analyses, that the symmetric and asymmetric cis-MoO2 stretches of Mo 16 O2(L) were at ca. 914 and 898 cm -1 .The corresponding bands for W 16 O2(L) were found at ca. 934 and 899 cm -1 .In the 18 O substituted derivatives, the ν(MO2)s were shifted to the lower energy by 41 cm -1 for Mo 18 O2(L) and by 43 cm -1 for W 18 O2(L) whereas the corresponding asymmetric stretches shift ca.39 and 28-37 cm -1 .The rather exhaustive vibrational analyses demonstrate that it is often reasonable to use complementary spectroscopic methods in order to avoid the misinterpretation of spectroscopic data.

FULL PAPER
The 16 O-and 18 O-enriched dioxidotungsten VI andmolybdenum VI complexes with a tetradentate amine phenolate ligand were prepared, whereas their IR and Raman spectra were compared with the complexes having a natural isotope distribution.The clear isotopic shifts to the lower energy were seen in the symmetric and asymmetric stretches attributed to the cis-MO2 moiety.The DFT theoretical calculations support the experimental results.

Figure 1 .
Figure 1.The molecular structure of the studied complexes.
18 O enriched complexes by DFT.The experimental single crystal Xray derived structures of Mo 16 O2(L) and W 16 O2(L) were first normalized in regard to the C-H distances and then the full geometries were relaxed at the RI-PBE/def2-TZVP level of theory.The optimized geometrical parameters are, at large, in good agreement with the experimental single crystal X-ray bond structures of the complexes.Only the M-N bond lengths deviate slightly from experimental values as they are overestimated by roughly 0.1 Å by the RI-PBE/def2-TZVP method.The optimized
18 O labelled dioxidometalVI  complexes MO2(L) (M = W, Mo) with a linear, tetradentate amine phenolate ligand (L) were prepared and their IR and Raman spectra were compared with the complexes having a natural isotope distribution.WO2(L) was made by the reaction of tungsten(VI) alkoxide W(eg)3 (eg = ethylene glycolate dianion) with an amine bisphenol ligand in the presence of either H2[ 16 O] or H2[ 18 O].In both syntheses, the compounds were isolated as pure isotopologues.The formation of two isotopologues confirms that the formation of the oxido ligands is due to the hydrolytic cleavage of the alkoxide bonds instead of possible ether elimination.This reaction provides an easy route to the 17 O or 18 O dioxido-labelled dioxidotungsten VI complexes.On the other hand, Mo 16 O2(L) was made of sodium molybdate and converted to Mo 18 O2(L) by ligand substitution using H2[ 18 O] as an oxide source, which demonstrates the different chemical reactivity of structurally identical W and Mo complexes.